Reversible gel compositions of polyvinyl alcohol and hydroxy aromatic compounds and their preparation



Patented July 15, 1941 REVERSIBLE GEL COMPOSITIONS OF POLY- VINYLALCOHOL AND HYDROXY ARO- MATIC COMPOUNDS AND THEIR PREPA- RATIONWendell. H. McDowell and William 0. Kenyon, Rochester, N. Y., assignorsto Eastman Kodak .Company, Rochester, N. Y., a corporation of New JerseyNo Drawing. Application February 12, 1940, Se-

rial No. 318,551. In Great Britain February 9 Claims. (Cl. 252-316) Theinvention relates to the preparation of reversible gels by gellingpolyvinyl alcohol with an unsubstituted hydroxy aromatic compound andthe reversible gels prepared therefrom.

The term "polyvinyl valcoho as used here n is to be understood asreferring to-peI-ymers in which at least 50% thereof-is composed ofvinyl alcohol constituent. It refers to polymers all of which arecomposed of vinyl alcohol units and also to polymers containing not onlyvinyl alcohol units but also vinyl acetate (and/or propionate and/orbutyrate) units providing the vinyl alcohol units make up at least 50%of the polymer.

Ordinarily polyvinyl alcohol is a water-soluble material and does notshow the properties of a gel under ordinary conditions. Therefore,polyvinyl alcohol does not usually find use where continuous contactwith water is involved as it has no permanence in such a situation.

One object of our invention is to treat polyvinyl alcohol with a hydroxyaromatic compound so as to convert it into a rigid gel which hasthermal-reversible properties. Another object of our invention is toprepare a product from polyvinyl alcohol, having gel characteristics, sothat it may be used instead of gelatin or other like materials for thevarious purposes in which those materials are used.

We have found that when polyvinyl alcohol alpha-naphthol or dihydricnaphthol, or their mixture, the polyvinyl alcohol forms an opaque whitegel which is firm and strong. We have found that the product so preparedis reversible,

remove gelling agent which is not bound to or adsorbed by the polyvinylalcohol. If the gel is subjected to so much washing that some of therequired gelling agent is removed an additional amount of gelling agentmay be added to the gel .by warming and then allowing to set is mixedwith the required amount of an unsubstituted dihydric phenol, trihydricphenol,-

again by cooling. We have found that polyvinyl cases in dilute alkalibefore adding thereto. Upon standing 3. polyvinyl alcohol gel is formedwhich can be redispersedupon heating.

The proportion of gelling agent employed may be varied depending on theconditions present. For instance, polyvinyl alcohol has been gelled withgelling agents specified herein in proportions from 2% up (based on theweight of the polyvinyl alcohol) in preparing gels in' accordance withour invention, although with the lower proportions of gelling agents,soft gels are obtained. The proportion of gelling agents, which is mostdesirable to use, depends on the temperature of gelling desired for theproduct formed, the viscosity of the polyvinyl alcohol, theconcentration of the polyvinyl alcohol and the particular gelling agentwhich is used. With higher viscosity polyvinyl alcohols, theproportionof gelling agent need not be as great as with the lowerviscosity polyvinyl alcohols to cause gelling within a time which wouldbe practical.

The proportion of gellingagent also depends on the agent itself and onthe hardness or firmness desired for the gel. For instance, if apolyvinyl alcohol having a molecular weight of 19,800 (a value obtainedfrom viscosity measurements) is used-in an aqueous solution containing5.6% v

by weight of polyvinyl alcohol, and 8.8% of alpha-naphthol, based on theweight of the polyvinyl alcohol is added thereto and the mass is allowedto stand, a firm opaque, white gel, is formed which forms a sol uponheating but gels again at 20 C. If only 4.4% of alpha-naphthol is used,a gel is obtained at a lowered temperature between 11 and 15 C. If only2.2% of alpha-naphthol is used, a gel is obtained at approximately 3 C.'With less than 2% of alphanaphthol, no appreciable gelling occurs at atemperature of 0 C. or more. The gels prepared using the-lower amountsof gelling agent are softer, the degree of firmness depending on theproportion of the particular gelling agent used.

A gelling agent, such as resorcinol, is effective but requires a largerproportion to cause the same degree of, gelling as alpha-naphtliol. Forinstance, if an aqueous solution of polyvinyl alcohol (prepared frompolyvinyl acetate having a viscosity of 15 secs.) containing 7%ofpolyvinyl alcohol is mixed with 50% of resorcinol (based on the weightof the polyvinyl alcohol) a rigid opaque gel is quickly obtained at roomtemperature. With the use of less resorcinol, such as down to 25%, thegelling is naturally not as rapid while with a greater proportion ofgelling agent, the gelling occurs more rapidly. With a lesseroncentration of polyvinyl alcohol, other things for the same speed ofgelling. Also, with the use of polyvinyl alcohols prepared from lowerviscosity. polyvinyl acetates, other things being ing equal, it isdesirable, as a rule, that a greater concentration of gelling agent beusedequal, the use of a greater concentration of gelling agent isdesirable to give the same speed of gelling or, as an alternative, themass may be cooled to facilitate the formation of the gel.

Where the purpose is to form a firm, rigid gel of polyvinyl alcohol atroom temperature using an aqueous solution of polyvinyl alcohol(molecular weight 19,800) of 5.6% concentration the proportion of thegelling agent employed should beat least roughly in the followingproportions, based on the weight of the polyvinyl alcohol.

' Per cent Dihydric phenols 40 Trihydric phen Alpha-naphfhnl 8bihydroxy-naphthalenes 3.5

be employed. For purposes of economy the use of not more than 200%,based on the weight of polyvinyl alcohol, is ordinarily preferred. For

convenience of operation, the range of 3-7% for the concentration ofpolyvinyl alcohol in water is preferred for the polyvinyl alcoholsusually used. These polyvinyl alcohols are ordinarily of a viscosity.which gives a molecular weight of approximately 6700-19300. With higherviscosity polyvinyl alcohols concentrations of less than 3% might bedesirable, while with lower viscosity polyvinyl alcohols, aconcentration of more than 1% might be employed.

The presence of alkali in the gelling of polyvinyl alcohol with hydroxyaromatic compounds decreases the efliciency of the gelling action.Therefore it is preferred that alkali be absent thol or theunsubstituted dihydric naphthols, such as! 1,4 dihydroxy naphthalene 1,7dihydroxy naphthalene 1,8 dihydroxy naphthalene 2,6 dihydroxynaphthalene 2,7 dihydroxy naphthalene 2,3 dihydroxy naphthalene 1,3dihydroxy naphthalene The following examples illustrate the gelling ofpolyvinyl alcohol with an unsubstituted hydroxy aromatic compound inaccordance with our invention:

' Example I A solution of .2 gram of catechol dissolved in 5 cc. ofwater was stirred into 5 cc. of a 5% aqueous solution of a polyvinylalcohol of medium viscosity. The solution was allowed to stand and in ashort time the formation of a rigid white gel was noted. If ahomogeneous solution is not obtained, such as might result frompremature separation out of the gel, the system should be warmed such asto about 40 C. and the suspended gel may thereby be redissolved. Anotherprocedure to assure avoidance of premature gelling is warming thesolutions before mixing them together and then causing the formation ofa gel by lowering the temperature. A gel was in gelling polyvinylalcohol in accordance with our invention. The presence of an excessiveamount of alkali prevents gelling. The amount of alkali present, if any,should be considerably less than that which will combine with all of thehydroxyl groups of the hydroxy aromatic compounds.

The monohydric phenols which are ordinarily regarded as representativeof the hydroxy aromatic compounds have no gelling action on polyvinylalcohol and, therefore, it would be expected that all of the hydroxyaromatic compounds are unsuitable for this purpose. However, we havefound that the following hydroxy aromatic compounds are effectivegelling agents for polyvinyl alcohol when mixed therewith in the amountsindicated herein: The unsubstituted dihydric phenols, such asresorcinol, catechol and obtained in this instance which formeda solupon heating and upon cooling the gel again formed.' Regardless of thenumber of times of heating and cooling, the formation of sol and gelwill take place each time.

Example II Example I wasrepeatedexcept that resorcinol was employedinstead of catechol. A white,

opaque gel quickly formed which redispersed upon heating the solution;Upon cooling, the gel was again obtained.

Example III Example I was repeated except that hydroquinone was employedinstead of catechol. An opaque gel was obtained by allowing the mass tostand at room temperature, but a longer time was required for gelformation thanin Examples 1 and II. The gel was reversible in that itformed a sol at elevated temperatures and reverted to a gel again uponcooling.

Example IV A hydrolyzed polyvinyl acetate composed of 18.6% vinylacetate and 81.4% vinyl alcohol was dissolved in warm water andresorcinol was added until the resin precipitated from solution. Uponcooling a stiff gel formed which dedissolved on heating. Coatings fromthis composition were unusually flexible.

Example V 1.2 grams of phloroglucinol was added to an aqueous solutioncontaining 5.6% by weight of polyvinyl alcohol of medium viscosity,while the solution was at a temperature of 70 C. The mass was examinedafter it had cooled to room Example VII .6 gram of 2,7-dihydroxynaphthalene was added to an aqueous solution containing 5.6%

by weight of polyvinyl alcohol'of medium viscosity, while the solutionwas at a temperature of 90 C. The mass was examined after it had cooledto room temperature and a firm, white, reversible gel was obtained. Theprocedure was repeated using .1 gram of the 2,7 dihydroxynaphthalene.The mass was found to gel at room temperature. a

The gels obtained in accordance with ourinvention may be used forvarious purposes. For instance, they may be used for photographicemulsions as the protective colloid for silver halide as described andclaimed in Lowe application Serial No. 318,559 filed of even date. whencolored, and in the form of sheeting, these gels may be used forsafelights or for light filters in photographic apparatus. Other uses towhich these gels may be put are subbinglayers in photographic film,cvercoatings or backings for film, as adhesives, thickening agents, orsizings for paper or cloth, or for use in coating compositions. Thesegels, particularly after they have been coated out into sheet form, maybe hardened such as by treating with formaldehyde, bichromates, chromealum or diketones. If desired, plasticizers may be incorporated in thegels, particularly if the formation of flexible sheeting therefrom iscontemplated. Some of the plasticizers which may be employed are'ethylene glycol, glycerol or monoacetin. The

sheeting may be prepared by coating out the sol on to a film-formingsurface and allowing'it to solidify or gel followed by drying.

As pointed out previously, gels-may be prepared using either polyvinylalcohol or polyvinyl compounds, containing a sufllcient proportion ofhydroxyl groups, so as to be a polyvinyl alcohol for all practicalpurposes. The criterion as to whether a hydrolyzed polyvinyl ester maybe employed as the polyvinyl alcohol is whether or not it is soluble inwater. If the polyvinyl ester containing a large proportion of hydroxylgroups is soluble in water, a rigid, opaque gel may be formed therefromin accordance with our invention.

All of the polyvinyl esters of the lower fatty acids containing at least50% of vinyl alcohol are suitable for use as the starting material inour invention. These water-soluble polyvinyl esters, such as polyvinylacetate may be prepared by hydrolyzing the polyvinyl ester withhydrochloric acid and water as described in U. 8. Patgelled polyvinylalcohol in the various uses given lends ltselfto standardization inthose proceses. The gels are obtained in 0paque f0rm in the gellingprocess-oi our invention but give clear coatings when dried in layerform.

We claim:

1. A firm, thermal-reversible gel composed of polyvinyl alcohol mixedwith a gelling amount of an unsubstituted hydroxy aromatic compoundselected from the group consisting of the unsubstituted polyhydric'phenols, unsubstituted a-hflPhthOl and 'the unsubstituteddihydricnaphthols.

2. A firm, thermal-reversible gel composed of .polyvinyl alcohol mixedwith a gelling amount of hydric naphthols at a temperature at which themixture retains its liquid form, and subsequently lowering thetemperature to a point at which the formation of a gel occurs.

6. A method of preparing a thermal-reversible gel which comprises mixingan aqueous solution of polyvinyl alcohol with a polyhydric phenol ingelling amount at a temperature at which the mixture retains its liquidform and subsequently lowering the temperature to the point at which theformation of a gel occurs.

7. A method of preparing a thermal-reversible gel which comprises mixingan aqueous solution of polyvinyl alcohol with a-naphthol in gelling entNo. 1,971,951 of Skirrow and Morrison until amount at a temperature atwhich the mixture the temperature to the point at which the formation ofa gel occurs.

9. .A method or preparing a thermal-reversible gel which comprisesmixing an aqueous solution of polyvinyl alcohol with 50-100% (based onthe weight of the polyvinyl alcohol) of a gelling agent selected fromthe group consisting of the unsubstituted polyhydric p h e n o l sunsubstituted a-naphthol and the unsubstituted dihydroxynaphthalenes ata temperature at which the mixture retains its liquid form andsubsequently low- .ering the temperature to a point at which theformation of a gel occurs.

WENDEIL H. MODOWEIL. WIILIAM O. KENYON.

